In integrated optics, the basic element is a light guide; it is formed by a guiding layer sandwiched between two substrates having a lower refractive index than that of the guiding layer; the guiding layer can be flat and of homogeneous thickness or else have a certain configuration (drawings, motifs). The geometry of the guiding layer is a function of the subsequent use of the light guide.
At present a number of techniques are used for producing light guides of flat geometry or having a particular profile; they can be grouped into two categories.
The first category relates to the surface modification of a transparent substrate, generally of glass, silica or lithium niobate. It relates more particularly to obtaining in the substrate a refractive index modified by a diffision of ions in the substrate.
The diffusion can be formed either under the impulse of a difference in chemical potential, or following an ionic implantation or a thermal diffusion of a dopant. In the latter case it is difficult to master a gradient of ordinary index, having regard to the concentration profiles of the doping agents in the substate.
Moreover, since the number of dopants compatible with the substrates generally used is limited, the values of the difference in index which can be obtained between the diffused layer and the substrate are limited.
The ordinary index gradient can readily be mastered with ionic implanation. However, this method is complex, requiring the use of high energy ion beams and a number of stages of annealing of the substrate to remedy the damage caused by implantation.
The second category, to which the process according to the invention belongs, relates to depositing thin layers of a transparent substrate. Unfortunately, the conventional depositing methods often produce thin layers of generally mediocre optical quality.
Moreover, for the manufacture of integrated optical components and opto-electronic components, the intention is progressively growing of replacing the materials generally used, such as glass, silica or lithium niobate by organic materials, mainly for reasons of flexibility of the materials.
Research in recent years has in fact shown that numerous organic materials in a crystallized form, more particularly polymers, might have non-linear optical responses, as well as electro-opitical responses, comparable to those of the best materials normally used. We may refer more particularly to the article by J. Badan et al. entitled "Non-linear Organic Crystals: theoretical concepts, materials and optical properties", published in ACS Symposium, Series 233, 81 (1983), pages 81-107 and the article by J. Zyss entitled "A Molecular Engineering Approach Towards the Design of Efficient Organic Crystals for Three Wave Mixing", published in Current Trends in Optics, 1981, pages 123-134.
Among these organic compounds in crystallized form, a large number of intramolecular charge transfer compounds are found which have the great advantage of having an aptitude for polarization, causing an optically active behaviour of the organic material such as, for example a doubling of the frequency of the light wave arriving on such material. With these crystallized organic materials it is also possible to obtain a phase modulation based on the Pockels effect, having regard to the opto-electronic properites of certain of these organic materials and the stability of their cystalline lattice.
Having regard to these new materials, the conventional techniques for depositing a thin crystalline layer on a transparent substrate, to produce a light guide of flat geometry or having a particular design, have a certain number of specific drawbacks.
The first prior art technique for depositing a thin crystalline layer of organic material on a substrate is epitaxy on the substrate from supersaturated solutions of organic material. It is described more particularly in an article in "Optical and Quantum Electronics", 7, (1975), pages 465-473 of H. P. Weber et al. entitled "Organic Materials for integrated Optics".
A multi-component optical system--i.e., one comprising a number of components--can be produced only with difficulty by this technique, which enables thin monocrystalline layers of large surface to be obtained at a temperature close to ambient temperature. This is due to the fact that the epitaxy solvent acts at one and the same time as an intermediary and an impurity. Moreover, it is difficult to produce patterns or a layer having a particular profile.
The second method of depositing a thin crystalline layer of organic material is the method of Langmuir-Blodgett. It was described more particularly in an article in the "Journal of Non-Crystalline Solids" 47, 2 (1982), pages 159-174 by C. W. Pitt, entitled "Materials and Fabrication Techniques for Integrated Optics: organic and amorphous materials".
This method, which allows the layer-by-layer depositing of an organic monocrystalline dielectric material (so-called monomolecular layers) ensures satisfactory mastery of the total thickness of the layer of organic material deposited. Moreover, the refractive index of these organic layers can be potentially changed in a precise manner, thus readily enabling patterns and light guides of flat geometry to be obtained. Moreover, the organic layers obtained can diffuse light, this phenomenon being due to the nucleation of holes, of the same dimension as the wave length of the light received by such layers.
Unfortunately, a number of constraining elements reduce the major advantages of this method. The first element is the compulsory presence in the molecule of organic material of a solventophilic-solventophobic functional pair, more particularly a hydrophilic-hydrophobic pair, which considerably limits the number of organic materials which can be deposited by this method on a substrate; this is all the truer, if we envisage depositing organic compounds having non-linear optical responses and/or electro-optical responses.
Moreover, the experimental conditions of growth of the layer of organic material on the substrate are relatively complex, since a sub-phase, a purified atmosphere and an anti-vibratile system must be used, and parameters such as the temperature and pH of the solution, the rate of deposit of the material and the variation in pressure of the surrounding atmosphere must be strictly controlled.
A third prior art method of depositing a thin crystalline layer of an organic material is evaporation in vacuo by catholic atomization. It is more particularly described in the already quoted article by C. W. Pitt.
With this method it is difficult to control the thickness of the deposited layer to form patterns with the layer. Moreover, such deposited organic layer age poorly, such ageing being mainly connected with the presence of free projected radicals in the thin layers during their production.
A last prior art method of depositing a thin crystalline layer of organic material is thermal evaporation. It is described more particularly in the article "High Purity Organic Molecular Crystal" by N. Karl in "Crystals" No. 4, (Springer Verlag) page 65, (1980).
This technique enables thin enough, very flat crystalline layers of organic material to be obtained, whatever the chemical nature of such materials may be. Unfortunately, the main disadvantages of this technique lie in the impossibility of operating with organic materials whose vapour tension is very high and difficult to control, the need to work in vacuo, and certain difficulties in obtaining thin layers of a given profile.
In this thermal evaporation method, evaporation is generally performed from a crucible or a Knudsen cell containing the material to be deposited in pulverulent form. Controlling evaporation means that the organic material must be brought to its melting point, so that a hot furnace must be used, and the material must be kept in the molten state throughout its growth on the substrate. This causes the risk of degrading the organic materials, which are sensitive to heat, and contaminating the molten material by the crucible, due to contact between the bath and the crucible.